Langford’s problem is named after the Scottish mathematician C. Dudley Langford who once observed his son playing with colored blocks. He noticed that the child had arranged three pairs of colored blocks such that there was one block between the red pair, two between the blue pair, and three between the yellow pair. He immediately generalized the problem from colours to numbers and to longer sequences.
Martin Gardner’s columns from Scientific America on Langford’s problem. He revisits the problem in Mathematical Magic Show, published by Alfred A. Knopf, ISBN 0-88385-449-X, first and second editions.
Eric Weisstein’s World of Mathematics entry on Langford’s problem from Wolfram Research.